We just moved into a new to us house, built in the 70's, and it has a metal roof.

The roof looks awful. It's rusted a little in some areas and just overall not nice looking. What would be a good way to "clean" this or make it look decent again? Sand blasting? Could I paint it? I've never seen a painted metal roof that I can remember.

Well,
The first thing you never want to do is sand blast the roof, it tends to leak with all the new holes.
Sanding is an option,just be careful up there when you do it or you will slide right off with all that fresh dust.
Try a company that refinishes roofs and see what suggestions you may get from the"experts."
If its to much woork or the roof is in worse shape than you describe, reroof.

I wonder if you are talking about a standing seam metal roof which is made on the site and installed; or is it a manufactrued roof like corrugated or five vee; or the new heavy steel roofing that comes in full size panels to reach from ridge to overhang? Any of them can and should be painted especially if there is rust present. It will add years to the life of the roof. There is a product for sandblasting the rust without severe damage to the metal; its called "black silk" and is actually pulverized coal. I would not paint over the rust because it will remain active even under the paint. You would soon have holes in the roofing metal; that's what we call "leaking like a sieve".